Optical device having a substrate and a laser unit that emits light into the substrate

ABSTRACT

An optical device includes a first substrate, having first and second surfaces, and a second substrate having a third surface. The first substrate includes: a laser unit, having an active layer and emitting light into the first substrate from the active layer; a reflecting mirror, having a plane obliquely intersecting an optical axis of light emitted from the laser unit, and being formed on the first surface so as to reflect the light toward the second surface; and a convex lens, being formed in a region on the second surface, the region including an optical axis of the light reflected by the reflecting mirror. The second substrate is provided with a grating coupler and an optical waveguide on the third surface, the optical waveguide having light incident on the grating coupler propagating therethrough.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority from Japanese application JP2012-262503 filed on Nov. 30, 2012, the content of which is herebyincorporated by reference into this application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical device.

2. Description of the Related Art

Interconnection capacity within an information device such as a routeror a server is increasing yearly. In the related art, althoughelectrical interconnection has been used, it is preferable to useoptical interconnection in order to realize further capacityenlargement. As one of techniques for realizing optical interconnection,silicon photonics is being developed. In order to realize the siliconphotonics, a material which is capable of crystal growth on a siliconsubstrate and capable of laser oscillation is desired, but such amaterial is not known on a practical level.

SUMMARY OF THE INVENTION

In silicon photonics, the integration of a light source is a majorproblem. In the related art, a light source for silicon photonics isdisclosed in US 2006/0239612 A1. The light source is an optical devicein which a Group III-V semiconductor laser having a reflecting mirror ona surface thereof is flip-chip mounted on a silicon substrate to becoupled to a grating coupler on the silicon substrate. However, in thisstructure, since emission light from a laser spreads out, it is notpossible to couple the emission light from the laser to the gratingcoupler with high efficiency. It is possible to parallelize emissionlight from a laser by interposing a lens between a surface emission typelaser and a grating coupler. However, it is difficult to form the lenson the same surface side of a substrate. This is because an ordinarylaser is formed to have a depth of approximately 2 μm from a surface,whereas the depth of the lens reaches 15 μm. For this reason, the lensis formed separately from the laser, which leads to a difficulty inperforming mounting position adjustment between a laser unit and thelens with a high level of accuracy, to an increase in manufacturingcosts, and to inhibition of miniaturization in an optical deviceincluding the surface emission type laser and the lens.

The present invention is contrived in view of such situations, and anobject thereof is to provide an optical device which includes asemiconductor laser, a lens, and a grating coupler and is easilymanufactured with a small number of components.

(1) In order to solve the above-mentioned problem, an optical deviceaccording to the present invention includes a first substrate and asecond substrate. The first substrate is formed of a semiconductormaterial, has a first surface and a second surface which is a rearsurface of the first surface. The first substrate includes a laser unitwhich has an active layer laminated between the first surface and thesecond surface and emits light into the first substrate from the activelayer; a reflecting mirror which has a plane obliquely intersecting anoptical axis of light emitted from the laser unit and propagating, andis formed on the first surface so as to reflect the light emitted fromthe laser unit and propagating, toward the second surface; and a convexlens which is formed in a region on the second surface, the regionincluding an optical axis of the light reflected by the reflectingmirror. The second substrate has a third surface facing the secondsurface, and is provided with a grating coupler and an optical waveguideon the third surface thereof, the optical waveguide having lightincident on the grating coupler propagating therethrough.

(2) In the optical device according to (1) mentioned above, the convexlens may converge or parallelize the light reflected by the reflectingmirror so as to cause the light to reach the grating coupler.

(3) In the optical device according to (2) mentioned above, the opticalaxis of the light reflected by the reflecting mirror may be shifted toan opposite side to the optical waveguide side of the second substratefrom an axis penetrating a center of the convex lens, and may penetratea surface of the convex lens.

(4) In the optical device according to (2) or (3) mentioned above, anangle between the optical axis of the light and a normal line of thereflecting mirror may be greater than 45 degrees, wherein the light isemitted from the laser unit, propagates, and reaches the reflectingmirror.

(5) The optical device according to any one of (1) to (4) mentionedabove may further include an isolator which is disposed between thefirst substrate and the second substrate and on a light path of light,to reach the grating coupler after having passed through the convexlens.

(6) The optical device according to (1) mentioned above may furtherinclude a first angle variable reflecting mirror, located between thefirst substrate and the second substrate, which reflects light havingpassed through the convex lens; and a second angle variable reflectingmirror which reflects the light reflected by the first angle variablereflecting mirror so as to cause the light to reach the grating coupler.

(7) The optical device according to (6) mentioned above may furtherinclude an isolator which is disposed on a light path of the light, toreach the second angle variable reflecting mirror after being reflectedby the first angle variable reflecting mirror.

(8) In the optical device according to (3) mentioned above, a shift ofthe optical axis of the light reflected by the reflecting mirror withrespect to the axis penetrating the center of the convex lens may beequal to or less than 18 μm.

(9) In the optical device according to (4) mentioned above, the formedangle may be equal to or less than 48 degrees.

(10) In the optical device according to (1) mentioned above, the firstsubstrate may include a plurality of integrated laser element unitshaving the laser unit, the reflecting mirror, and the convex lens. Aplurality of integrated waveguide circuit units including the gratingcoupler and the optical waveguide may be formed in the second substrate.Light having passed through the convex lens of each of the integratedlaser element units may be incident on the grating coupler of thecorresponding integrated waveguide circuit unit.

(11) The optical device according to (10) mentioned above may furtherinclude an isolator which is disposed between the first substrate andthe second substrate and over light paths of light beams, to reach thegrating couplers of the corresponding integrated waveguide circuit unitsafter having passed through the convex lenses of the plurality ofintegrated laser element units.

(12) In the optical device according to (1) mentioned above, the firstsubstrate may include a plurality of integrated laser element unitshaving the laser unit and a laser optical waveguide for propagating thelight emitted from the laser unit to the reflecting mirror. Wavelengthsof the light beams emitted from the laser units of the plurality ofintegrated laser element units may be different from each other. Opticalaxes of light beams, propagating through the laser optical waveguides ofthe plurality of integrated laser element units and reflected by thereflecting mirror, may be shifted to an opposite side to the opticalwaveguide side of the second substrate from an axis penetrating a centerof the convex lens, and may penetrate a surface of the convex lens.

(13) In the optical device according to anyone of (1) to (12) mentionedabove, the second substrate may be formed of any one material of Si,GaAs, InP, and glass.

According to the present invention, provided is an optical device whichincludes a semiconductor laser, a lens, and a grating coupler and iseasily manufactured with a small number of components.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a top view of an optical device according to a firstembodiment of the present invention.

FIG. 1B is a cross-sectional view of the optical device according to thefirst embodiment of the present invention.

FIG. 1C is a top view of a silicon substrate of the optical deviceaccording to the first embodiment of the present invention.

FIG. 2 is a cross-sectional view of an optical device according to asecond embodiment of the present invention.

FIG. 3 is a diagram illustrating a relationship between a shift d ofincident light on a convex lens and an incidence angle Θ_(in) of lighton a grating coupler.

FIG. 4 is a cross-sectional view of an optical device according to athird embodiment of the present invention.

FIG. 5 is a diagram illustrating a relationship between an incidenceangle Θ_(in) of light on a reflecting mirror and an incidence angleΘ_(in) of light on a grating coupler.

FIG. 6A is a top view of an optical device according to a fourthembodiment of the present invention.

FIG. 6B is a cross-sectional view of the optical device according to thefourth embodiment of the present invention.

FIG. 7 is a cross-sectional view of an optical device according to afifth embodiment of the present invention.

FIG. 8A is a top view of an optical device according to a sixthembodiment of the present invention.

FIG. 8B is a top view of a silicon substrate of the optical deviceaccording to the sixth embodiment of the present invention.

FIG. 9A is a top view of an optical device according to a seventhembodiment of the present invention.

FIG. 9B is a cross-sectional view of the optical device according to theseventh embodiment of the present invention.

FIG. 9C is a cross-sectional view of the optical device according to theseventh embodiment of the present invention.

FIG. 10 is a diagram illustrating a relationship between an incidenceangle Θ_(in) of light on a grating coupler and a wavelength λ.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be describedspecifically and in detail with reference to the accompanying drawings.Meanwhile, in all diagrams for illustrating the embodiments, membershaving the same function are denoted by the same reference numeral, anda description thereof will not be repeated. In addition, the followingdiagrams are merely for illustrating examples of the embodiments, andthe size of the diagram and a reduced size described in this example arenot necessarily consistent with each other.

First Embodiment

FIG. 1A is a top view of an optical device according to a firstembodiment of the present invention. The optical device according tothis embodiment includes a first semiconductor substrate (firstsubstrate) and a second semiconductor substrate (second substrate). Thefirst semiconductor substrate (semiconductor laser element 1) includes aDFB laser unit 2, a reflecting mirror 3, and a convex lens 4. That is,the semiconductor laser element 1 is a semiconductor optical device inwhich the DFB laser unit 2, the reflecting mirror 3, and the convex lens4 are integrated on the first semiconductor substrate. Meanwhile, theDFB laser, which is a distributed-feedback laser, is formed on an InPsubstrate. That is, here, a semiconductor material of the firstsubstrate is InP, but the present invention is not limited thereto. Thefirst substrate may be a substrate formed of another semiconductormaterial. In addition, the second semiconductor substrate is a siliconsubstrate 21, and a grating coupler 22 and an optical waveguide areformed on a surface of the silicon substrate 21 as described later.

FIG. 1B is a cross-sectional view of the optical device according tothis embodiment, and shows a cross-section along line IB-IB illustratedin FIG. 1A. The first semiconductor substrate has a first surface whichis an upper surface illustrated in FIG. 1B, and a second surface whichis a lower surface illustrated in FIG. 1B, and the second surface is arear surface with respect to the first surface. The DFB laser unit 2includes an active layer 11 that generates a gain, and a diffractiongrating 12. The active layer 11 is laminated between the first surfaceand the second surface, and emits light into the first semiconductorsubstrate from an end of the active layer 11. In the first semiconductorsubstrate (semiconductor laser element 1), an upper electrode 13(surface electrode) is formed on the first surface of the DFB laser unit2, and a lower electrode 14 (rear electrode) is formed on the secondsurface thereof, and thus it is possible to inject a current into theactive layer 11 by the upper electrode 13 and the lower electrode 14.

The light emitted into the first semiconductor substrate from the end bythe DFB laser unit 2 propagates through the first semiconductorsubstrate in a right direction of FIG. 1B and reaches the reflectingmirror 3. The reflecting mirror 3 is configured such that asemiconductor positioned below the first surface (upper surface) of thefirst semiconductor substrate is removed, and that a plane obliquelyintersecting the optical axis of the light reaching the reflectingmirror 3 is formed. That is, the reflecting mirror 3 is formed on thefirst surface, and the plane of the reflecting mirror 3 is a portion ofthe first surface. A normal line of the plane of the reflecting mirror 3obliquely interests the optical axis of the light reaching thereflecting mirror 3 at 45 degrees, and thus the reflecting mirror 3reflects the light in a downward direction of FIG. 1B. That is, thereflecting mirror 3 reflects the light toward the second surface (lowersurface).

The light reflected by the reflecting mirror 3 propagates through thefirst semiconductor substrate in a downward direction of FIG. 1B andreaches the convex lens 4. The convex lens 4 is configured such that asemiconductor positioned below the second surface (lower surface) of thefirst semiconductor substrate is removed, and that a convex curvesurface is formed. The convex lens 4 is formed in a region, which is thesecond surface, including the optical axis of the light reflected by thereflecting mirror 3. The convex lens 4 converges or parallelizes thelight reflected by the reflecting mirror 3 so as to cause the light toreach the grating coupler 22 which is to be described later. In FIG. 1B,the optical axis of the light emitted from the DFB laser unit 2,reflected by the reflecting mirror 3, passing through the convex lens 4,and reaching the grating coupler 22 is illustrated as a broken line, andthe optical axis of the light passing through the convex lens 4penetrates the center of the convex lens 4. Here, the diameter of theconvex lens 4 is 90 μm, and the radius of curvature of a lens surface is125 μm. The semiconductor laser element 1 is manufactured using, forexample, a manufacturing method disclosed in JP 2008-277445 A.

FIG. 1C is a top view of the silicon substrate 21 (second semiconductorsubstrate) of the optical device according to this embodiment. Thesilicon substrate 21 has a third surface which is an upper surfaceillustrated in FIG. 1B, and the third surface is disposed so as to facethe second surface (lower surface) of the first semiconductor substrate.In the optical device according to this embodiment, the semiconductorlaser element 1 is directly mounted onto the silicon substrate 21. Thegrating coupler 22, a first optical waveguide 23, and a second opticalwaveguide 24 are formed on the third surface of the silicon substrate21. Meanwhile, the grating coupler 22 and the optical waveguides (firstoptical waveguide 23 and second optical waveguide 24) will becollectively referred to as an integrated waveguide circuit unit 25. Thegrating coupler 22 is an element for coupling light having passedthrough the convex lens 4 of the semiconductor laser element 1. Thefirst optical waveguide 23 is disposed so as to come into contact withthe right side of the grating coupler 22 of FIG. 1C, and has a shape(triangular shape) in which a width thereof decreases toward the rightside. The light coupled in the grating coupler 22 propagates through thefirst optical waveguide 23, and then propagates to the second opticalwaveguide 24 in a single mode. The second optical waveguide 24 disposedso as to come into contact with the first optical waveguide 23 andextending in a right direction of FIG. 1B and FIG. 1C is a waveguidethat satisfies a single mode condition to stably guide light. Althoughnot shown in the drawing, for example, a Mach-Zehnder (MZ) modulator, agrating coupler, and an optical fiber for transmitting light may beformed at the front where the second optical waveguide 24 extends.

The feature of the optical device according to the present invention isto include the semiconductor laser element 1 (first semiconductorsubstrate) having the reflecting mirror 3 and the convex lens 4 formedon the first surface and the second surface, respectively, and thesecond semiconductor substrate having the grating coupler 22 formed onthe third surface. The light emitted from the DFB laser unit 2 isconverged or parallelized by the convex lens 4 and then reaches thegrating coupler 22, and thus the light can be coupled in the gratingcoupler 22 with high efficiency. The reflecting mirror 3 and the convexlens 4 are formed on the surface of the first semiconductor substrate,and thus a reduction in a number of components is realized as comparedto a case where a reflecting mirror and a convex lens are formedseparately from a laser, thereby allowing the optical device to becreated through a simpler process. Therefore, it is possible to realizea reduction in manufacturing costs and miniaturization of the device.

Second Embodiment

An optical device according to a second embodiment of the presentinvention is different from that in the first embodiment in thearrangement of a convex lens 4, but the other structures are the same asthose of the optical device according to the first embodiment. In theoptical device according to the first embodiment, the axis penetratingthe center of the convex lens 4 is consistent with the optical axis ofthe light reflected by the reflecting mirror 3 and reaching the convexlens 4. In contrast, in the optical device according to this embodiment,an optical axis of light reflected by a reflecting mirror 3 (opticalaxis of incident light on convex lens 4) is shifted to the opposite sideto the optical waveguide side of a second semiconductor substrate froman axis which penetrates the center of the convex lens 4 (the opticalaxis of the convex lens 4), and penetrates the surface of the convexlens 4. Here, the phrase “the optical axis of the incident light on theconvex lens 4 is shifted to the opposite side to the optical waveguideside from the optical axis of the convex lens 4, and penetrates thesurface of the convex lens 4” as used herein is as described below. Theoptical waveguides formed on a third surface of the second semiconductorsubstrate are a first optical waveguide 23 and a second opticalwaveguide 24. The first optical waveguide 23 comes into contact with theright side of the grating coupler 22 of FIG. 1C, and the second opticalwaveguide 24 further extends to the right side of FIG. 1C. Therefore,this means that the optical axis of the incident light on the convexlens 4 is shifted to the left side of FIG. 1C from the optical axis ofthe convex lens 4 and penetrates the surface of the convex lens 4.Meanwhile, it is preferable that both the optical axis of the convexlens 4 and the optical axis of the incident light on the convex lens 4are included in a plane, perpendicular to the third surface, whichpenetrates the second optical waveguide 24. However, both or either ofthe optical axis of the convex lens 4 or the optical axis of theincident light on the convex lens 4 may not be included in such a plane.Even in such a case, the optical axis of the incident light on theconvex lens 4 may be shifted to the opposite side to the opticalwaveguide side from a plane, perpendicular to an extension direction ofthe second optical waveguide 24, which includes the optical axis of theconvex lens 4, and may penetrate the surface of the convex lens 4.

FIG. 2 is a cross-sectional view of the optical device according to thisembodiment, and corresponds to the cross-section along line IB-IBillustrated in FIG. 1A. An enlarged view of a region of FIG. 2 which issurrounded by a broken line is shown on the lower side. Similarly toFIG. 1A, light reflected by the reflecting mirror 3 propagates in adownward direction of FIG. 2 and reaches the convex lens 4. In theoptical device illustrated in FIG. 2, both the optical axis of theconvex lens 4 and the optical axis of the incident light on the convexlens 4 are included in a plane, perpendicular to the third surface,which penetrates the second optical waveguide 24. The optical axis(broken line) of the incident light is shifted to the left side from theoptical axis (solid line) of the convex lens 4, and the shift of theoptical axis of the incident light on the convex lens 4 is d. Theincident light on the convex lens 4 is shifted to the opposite side tothe optical waveguide from the center of the convex lens 4, and thus theoptical axis of the light having passed through the convex lens 4 istilted to the axis side (to the right side of FIG. 2) of the center ofthe convex lens 4 from the optical axis of the incident light by theconvex lens 4. Here, an angle between the optical axis of the lighthaving passed through the convex lens 4 and a normal line of the thirdsurface is set to an incidence angle Θ_(in).

FIG. 3 is a diagram illustrating a relationship between the shift d ofthe incident light on the convex lens 4 and an incidence angle Θ_(in) oflight on the grating coupler 22. When the light incident on the gratingcoupler 22 is tilted to the optical waveguide side in a normaldirection, the coupling efficiency is enhanced. In particular, when theincidence angle Θ_(in) is equal to or less than 20 degrees, the couplingefficiency is high. At this time, the shift d of the optical axis of theincident light is 18 μm, and the shift d is preferably equal to or lessthan 18 μm. From the viewpoint of coupling efficiency, the incidenceangle Θ_(in) is preferably equal to or less than 10 degrees±5 degrees(shift d is 5 μm to 14.5 μm), and is further preferably equal to or lessthan 10 degrees±2 degrees (shift d is 8 μm to 12 μm). Meanwhile, whenthe shift d is 10 μm, the incidence angle Θ_(in) is 10 degrees.

Third Embodiment

An optical device according to a third embodiment of the presentinvention has the same structure as the optical device according to thefirst embodiment except that a tilt of a plane of a reflecting mirror 3is different from that in the first embodiment. In the optical deviceaccording to the first embodiment, the normal line of the plane of thereflecting mirror 3 obliquely intersects the optical axis of the light(incident light on the reflecting mirror 3) which reaches the reflectingmirror 3 at 45 degrees. On the other hand, in the optical deviceaccording to this embodiment, an angle between an optical axis of thelight and a normal line of the reflecting mirror 3 is greater than 45degrees, wherein the light is emitted from the DFB laser unit 2,propagates, and reaches the reflecting mirror 3.

FIG. 4 is a cross-sectional view of the optical device according to thisembodiment, and corresponds to the cross-section along line IB-IBillustrated in FIG. 1A. An enlarged view of a region of FIG. 4 which issurrounded by a broken line is shown on the lower side. An angle betweenan optical axis of light incident on a plane of the reflecting mirror 3and the normal line of the reflecting mirror 3 is set to an incidenceangle Θ_(in) of light on the reflecting mirror 3. Herein, since theincidence angle Θ_(in) of the light on the reflecting mirror 3 isgreater than 45 degrees, the optical axis of the light reflected by thereflecting mirror 3 is tilted to the optical waveguide side from thenormal line of the third surface. Herein, the optical axis of the lightreflected by the reflecting mirror 3 penetrates the center of the convexlens 4, and thus an optical axis of the light having passed through theconvex lens 4 is consistent with the optical axis of the light reflectedby the reflecting mirror 3 without being refracted by the convex lens.Similarly to FIG. 2, an incidence angle of light on the grating coupler22 is set to an incidence angle Θ_(in).

FIG. 5 is a diagram illustrating a relationship between the incidenceangle Θ_(in) of the light on the reflecting mirror 3 and the incidenceangle Θ_(in) of the light on the grating coupler 22. Similarly to thesecond embodiment, when the light incident on the grating coupler 22 istilted to the optical waveguide side in a normal direction, the couplingefficiency is enhanced. In particular, when the incidence angle Θ_(in)is equal to or less than 48 degrees, the incidence angle Θ_(in) isgreater than 0 degrees and is equal to or less than 20 degrees, which ispreferable. From the viewpoint of coupling efficiency, the incidenceangle Θ_(in) is preferably equal to or less than 10 degrees±5 degrees(Θ_(in) is 45.8 degrees to 47.3 degrees), and is further preferablyequal to or less than 10 degrees±2 degrees (Θ_(in) is 46.2 degrees to46.8 degrees). Meanwhile, when the incidence angle Θ_(in) is 46.5degrees, the incidence angle Θ_(in) is 10 degrees, which is optimal.

Meanwhile, herein, the optical axis of the light reflected by thereflecting mirror 3 and incident on the convex lens 4 penetrates thecenter of the convex lens 4, but the present invention is not limitedthereto. Similarly to the second embodiment, the optical axis of thelight incident on the convex lens 4 may be shifted from the center ofthe convex lens 4 and penetrate the surface of the convex lens 4. Inthis case, the incidence angle Θ_(in) of the light on the gratingcoupler 22 is an angle in which a tilt caused by the reflection of thereflecting mirror 3 and a tilt caused by the refraction on the surfaceof the convex lens 4 are combined with each other in an overlappingmanner. In addition, it is preferable that an incidence surface createdby the optical axis of the light incident on the reflecting mirror 3penetrates the second optical waveguide 24, but the present invention isnot limited thereto. Even in this case, the optical axis of the light onthe grating coupler 22 may be tilted to the optical waveguide sidefurther than a plane, including a point penetrating the surface of thegrating coupler 22, which is perpendicular to the extension direction ofthe second optical waveguide 24.

Fourth Embodiment

An optical device according to a fourth embodiment of the presentinvention is different from those in the first to third embodiments inthat the optical device further includes an isolator 32 and accordinglyadditionally includes a holding member, but the other structures are thesame as the optical device according to any of the first to thirdembodiments. The isolator 32 is disposed between a semiconductor laserelement 1 (first semiconductor substrate) and a silicon substrate 21(second semiconductor substrate) and on a light path of light, to reacha grating coupler 22 after having passed through a convex lens 4.

FIG. 6A is a top view of the optical device according to thisembodiment. FIG. 6B is a cross-sectional view of the optical deviceaccording to this embodiment, and shows a cross-section along lineVIB-VIB illustrated in FIG. 6A. The semiconductor laser element 1 andthe silicon substrate 21 illustrated in FIG. 6B are the same as thesemiconductor laser element 1 and the silicon substrate 21 according tothe first embodiment. As illustrated in FIG. 6B, the optical deviceaccording to this embodiment further includes a laser submount 31, theisolator 32, and a U-shaped guide 33. The semiconductor laser element 1is mounted to the laser submount 31, and is fixed to the laser submount31 using an upper electrode 13 (surface electrode). The isolator 32 isconstituted by an isolator chip 34 and an isolator magnet 35, and theisolator chip 34 is fixed to the isolator magnet 35. The U-shaped guide33 and the isolator magnet 35 are fixed onto the silicon substrate 21,and the laser submount 31 having the semiconductor laser element 1mounted thereto is fixed to the U-shaped guide 33 and the isolatormagnet 35. That is, the isolator chip 34 of the isolator 32 is disposedbetween the convex lens 4 of the semiconductor laser element 1 and thegrating coupler 22 of the silicon substrate 21. Meanwhile, the lasersubmount 31, the U-shaped guide 33, and the isolator magnet 35 areholding members for fixing the semiconductor laser element 1 and theisolator chip 34.

In the optical device according to this embodiment, the isolator 32 isdisposed between the convex lens 4 and the grating coupler 22. Thus,even though reflected light propagates in a reverse direction from anoptical waveguide on the silicon substrate 21 or an optical member (forexample, optical fiber) before the optical waveguide, the isolator 32can reduce the reflected light, and thus the intensity of the reflectedlight incident on the semiconductor laser element 1 is significantlyattenuated, thereby exhibiting an additional effect such as a stabilizedoscillation state of the optical device. By the isolator 32 beingdisposed, a distance between the convex lens 4 and the grating coupler22 becomes longer than those in the first to third embodiments. Whenlight emitted from the semiconductor laser element spreads out, theelement size of the grating coupler is required to be increased with theincrease in distance. However, in this embodiment, light emitted fromthe convex lens 4 by the semiconductor laser element 1 is converged orparallelized by the convex lens 4, and thus the mode radius of lightincident on the grating coupler 22 is as small as approximately 20 μm.Therefore, even though light passes through the isolator 32 having athickness of approximately 1.5 mm, the element size of the gratingcoupler 22 is not required to be increased as compared with the case ofspread-out light, and thus the present invention has a pronouncedeffect.

Fifth Embodiment

An optical device according to a fifth embodiment of the presentinvention is different from that in the first embodiment in that theoptical device further includes an isolator 32 and two angle variablereflecting mirrors 41 and 42 and accordingly additionally includes aholding member, but the other structures are the same as the opticaldevice according to the first embodiment.

FIG. 7 is a cross-sectional view of the optical device according to thisembodiment. Both the angle variable reflecting mirrors 41 and 42 arelocated between a semiconductor laser element 1 (first semiconductorsubstrate) and a silicon substrate 21 (second semiconductor substrate).The angle variable reflecting mirror 41 (first angle variable reflectingmirror) is disposed on an optical axis of light having passed through aconvex lens 4 and reflects the light. The angle variable reflectingmirror 42 (second angle variable reflecting mirror) reflects the lightreflected by the angle variable reflecting mirror 41 and causes thelight to reach a grating coupler 22. Furthermore, the (isolator chip of)isolator 32 is disposed on a light path of the light, to reach the anglevariable reflecting mirror 42 after being reflected by the anglevariable reflecting mirror 41. The light reflected by the angle variablereflecting mirror 41 passes through the isolator 32 and is then incidenton the angle variable reflecting mirror 42. Silicon substrates 43 and 44for holding the angle variable reflecting mirrors 41 and 42,respectively, are fixed onto the silicon substrate 21. The semiconductorlaser element 1 is fixed to a laser submount 31 using a lower electrode14 (rear electrode), and the laser submount 31 having the semiconductorlaser element 1 mounted thereto is fixed to a top surface of the siliconsubstrate 43.

Both the angle variable reflecting mirrors 41 and 42 have a microelectromechanical systems (MEMS) structure, and are capable of adjustingan angle under its control. The light reflected by the second anglevariable reflecting mirror 42 is incident on the grating coupler 22formed on the silicon substrate 21. With respect to an incidence angleΘ_(in) of the light on the grating coupler 22, it is possible to performoptical-axis adjustment including angle adjustment by using the twoangle variable reflecting mirrors 41 and 42. Therefore, there arepronounced effects that the mounting is facilitated because fineadjustment is not necessary when mounting the semiconductor laserelement 1 to the optical device, and that it is also possible to performangle adjustment even after the semiconductor laser element 1 ismanufactured.

Herein, although the isolator 32 is disposed between the two anglevariable reflecting mirrors 41 and 42, the isolator 32 is notindispensable. In addition, in this embodiment, the two angle variablereflecting mirrors 41 and 42 and the isolator 32 are disposed, and thusthe light path between the convex lens 4 and the grating coupler 22 isincreased. However, in this embodiment, light emitted from the convexlens 4 by the semiconductor laser element 1 is converged or parallelizedby the convex lens 4. Therefore, similarly to the fourth embodiment, theelement size of the grating coupler 22 is not required to be increased,and thus the present invention has a pronounced effect.

Sixth Embodiment

An optical device according to a sixth embodiment of the presentinvention is different from that in the fourth embodiment in that asemiconductor laser and a grating coupler are configured as an array,but the other structures are the same as the optical device according tothe fourth embodiment. That is, a semiconductor laser element (firstsemiconductor substrate) includes a plurality (herein, four) ofintegrated laser element units each of which includes the laser unit,the reflecting mirror, and the convex lens. A plurality (herein, four)of integrated waveguide circuit units each of which includes a gratingcoupler and an optical waveguide, are formed on a silicon substrate 21(second semiconductor substrate). An isolator 32 is disposed between thefirst semiconductor substrate and the second semiconductor substrate andover light paths of light, to respectively reach the grating couplers ofthe integrated waveguide circuit units after having passed throughconvex lenses of the plurality of integrated laser element units.

FIG. 8A is a top view of the optical device according to thisembodiment. FIG. 8B is a top view of the silicon substrate 21 (secondsemiconductor substrate) of the optical device according to thisembodiment. When each of the integrated laser element units includingthe DFB laser unit, the reflecting mirror, and the convex lens is set toone array, a semiconductor laser element 51 according to this embodimentincludes a total of four arrays disposed lined up in a direction(vertical direction) crossing an emission direction of light of the DFBlaser unit. When each of the integrated waveguide circuit units,respectively corresponding to the integrated laser element units of thesemiconductor laser element 51, which includes the grating coupler andthe optical waveguides (first optical waveguide and second opticalwaveguide) is set to one array, a total of four arrays disposed lined upso as to be along the direction in which the integrated laser elementunits are disposed lined up are formed on a third surface of the siliconsubstrate 21. In FIG. 8A, four integrated laser element units 52A, 52B,52C, and 52D are illustrated together with four DFB laser units, fourreflecting mirrors 3A, 3B, 3C, and 3D, and four convex lenses 4A, 4B,4C, and 4D, but configurations of each of the integrated laser elementunits are the same as the configurations included in the semiconductorlaser element 1 according to any one of the first to third embodiments.

In FIG. 8B, four integrated waveguide circuit units 25A, 25B, 25C, and25D are illustrated together with four grating couplers 22A, 22B, 22C,and 22D, four first optical waveguides 23A, 23B, 23C, and 23D, and foursecond optical waveguides 24A, 24B, 24C, and 24D, but configurations ofeach of the integrated waveguide circuit units are the same as theconfigurations formed in the silicon substrate 21 according to the firstembodiment. Therefore, a cross-section along line VIB-VIB of the opticaldevice according to this embodiment which is illustrated in FIG. 8A isthe same as the cross-section of the optical device according to thefourth embodiment which is illustrated in FIG. 6B, except that thereference numerals of the semiconductor laser elements are differentfrom each other.

The (isolator chip of) isolator 32 is disposed over all of four lightpaths connecting the four convex lenses 4A, 4B, 4C, and 4D and the fourgrating couplers 22A, 22B, 22C, and 22D, respectively. Here, the numberof isolators 32 is one. Thus, even though reflected light propagates ina reverse direction from an optical waveguide on the silicon substrate21 or an optical member (for example, optical fiber) before the opticalwaveguide, the one isolator 32 can reduce the reflected light on all thefour light paths, and thus the intensity of the reflected light incidenton the semiconductor laser element 51 is significantly attenuated,thereby exhibiting an additional effect such as a stabilized oscillationstate of the optical device. The interval (distance between centrallines of adjacent DFB laser units) among the four DFB laser units is 100μm. Similarly to the optical device illustrated in FIG. 6B, the isolator32 is disposed between a laser submount 31 having the semiconductorlaser element 51 mounted thereto and the silicon substrate 21. At thistime, the number of isolators 32 is one, and the isolator 32 operates asan isolator with respect to the four light paths. The isolator 32(isolator magnet 34) has a size of 1 mm×1 mm, whereas the centers of thefour lenses are separated from each other at a maximum of 300 μm, andthe four arrays can share the one isolator 32.

In the optical device according to this embodiment, the four integratedlaser element units are integrated in the first semiconductor substrate.The convex lens is formed on a second surface of the first semiconductorsubstrate, and thus the diameter of the convex lens is 70 μm which issmaller than that of an ordinary glass lens. Therefore, the diameter ofthe convex lens is smaller than the interval between the DFB laserunits, and thus four convex lenses can be disposed lined up, therebyrealizing the optical device including four arrays. In this embodiment,a case in which four DFB laser units are included in the semiconductorlaser element 51 has been described as an example. However, as long asit is a case in which a plurality (two or more) of DFB laser units areincluded, the present invention is not limited thereto. The number ofisolators may be determined in consideration of the number of arrays tobe disposed and the element size of the isolator.

Seventh Embodiment

FIG. 9A is a top view of an optical device according to a seventhembodiment of the present invention. The optical device according tothis embodiment includes a first semiconductor substrate and a secondsemiconductor substrate. The first semiconductor substrate(semiconductor laser element 61) includes four DFB laser units 62A, 62B,62C, and 62D which are disposed lined up in a direction (verticaldirection) crossing an emission direction of light, a reflecting mirror63, a convex lens 64, and four laser optical waveguides 67A, 67B, 67C,and 67D which propagate light beams respectively emitted from the fourDFB laser units 62A, 62B, 62C, and 62D and cause the light beams toreach the reflecting mirror 63. Meanwhile, herein, when one DFB laserunit and the laser optical waveguide extending from an end of the DFBlaser unit are collectively referred to as an integrated laser elementunit, the first semiconductor substrate includes a plurality of theintegrated laser element units, the reflecting mirror 63, and the convexlens 64. The interval between the adjacent DFB laser units is 100 μm.The four DFB laser units 62A, 62B, 62C, and 62D emit light beams havinga wavelength λ of 1340 nm, a wavelength λ₂ of 1320 nm, a wavelength λ₃of 1300 nm, and a wavelength λ₄ of 1280 nm, respectively. That is, thefour DFB laser units 62A, 62B, 62C, and 62D emit light beams havingdifferent wavelengths. The four laser optical waveguides 67A, 67B, 67C,and 67D extend from ends of the four DFB laser units 62A, 62B, 62C, and62D, are curved so as to be collected in the reflecting mirror 63 inaccordance with the extension thereof, further extend while being linedup at equal intervals, and reach the reflecting mirror 63. That is, thelaser optical waveguide is an optical waveguide that propagates thelight emitted from the DFB laser unit to the reflecting mirror 63. Here,the interval between two laser optical waveguides adjacent to each otherin the vicinity of the reflecting mirror 63 is 2.5 μm which is smallerthan that on the DFB laser unit side.

FIG. 9B and FIG. 9C are cross-sectional views of the optical deviceaccording to this embodiment. FIG. 9B illustrates a cross-section alongline IXB-IXB illustrated in FIG. 9A, and FIG. 9C illustrates across-section along line IXC-IXC illustrated in FIG. 9A. Meanwhile, theline IXB-IXB illustrated in FIG. 9A is a cross-section that passesthrough the central line of the DFB laser unit 62A, further passesthrough the central line of laser optical waveguide 67A to be curved,and further linearly intersects the reflecting mirror 63, and thenreaches an end of the optical device. As illustrated in FIG. 9B, thelaser optical waveguide 67A is formed in a core layer 66, and thus lightpropagating through the laser optical waveguide can reach the reflectingmirror 63 with low loss. As illustrated in FIG. 9B, a normal line of aplane of the reflecting mirror 63 obliquely intersects optical axes 4 offour light beams reaching the reflecting mirror 63 at 45 degrees, andthe reflecting mirror 63 reflects the four light beams toward a secondsurface (lower surface). At this time, the optical axes of the fourlight beams pass through the center of the convex lens 64, are lined upin a straight line along the line IXC-IXC in FIG. 9A, and reach thesecond surface of the convex lens 64. The optical axes of the four lightbeams are shifted in order on the opposite side to the optical waveguideside from the center of the convex lens 64, and penetrate the surface ofthe convex lens 64. The light beam closest to the center of the convexlens 64 is a light beam emitted from the DFB laser unit 62D, propagatingthrough the laser optical waveguide 67D, and then reflected by thereflecting mirror 63, and is set to be 5.5 μm away from the center ofthe convex lens 64. The four light beams shifted in order from thecenter of the convex lens 64 are lined up in the order of light beamsemitted from the DFB laser units 62D, 62C, 62B, and 62A.

As illustrated in FIG. 9C, similarly to the first embodiment, a gratingcoupler 72, a first optical waveguide 73, and a second optical waveguide74 are formed lined up in order on a third surface of the siliconsubstrate 21, but are lined up in a direction perpendicular to anemission direction of the DFB laser unit rather than in a directionparallel thereto. Thus, the second optical waveguide 74 extends alongthe direction perpendicular thereto. The four light beams reflected bythe reflecting mirror 63 are lined up along the direction perpendicularthereto. In FIG. 9C, in the four light beams, a light beam emitted fromthe DFB laser unit 62A and propagating and a light beam emitted from theDFB laser unit 62D and propagating are only shown as λ₁ and λ₄,respectively, but two light beams emitted from the DFB laser units 62Band 62C and propagating are also actually present between two lightbeams. The four light beams are incident on positions shifted from thecenter of the convex lens 64, and consequently, are incident on thegrating coupler 72 at different angles. Here, incidence angles Θ_(in) ofthe four light beams on the grating coupler 72 are Θ_(in) (λ₁)=12.9degrees, Θ_(in) (λ₂)=10.4 degrees, Θ_(in) (λ₃)=8 degrees, and Θ_(in)(λ₄)=5.6 degrees, respectively, in the order (in the order from the leftside to the right side of FIG. 9C) of light beams respectively emittedfrom the DFB laser units 62A, 62B, 62C, and 62D.

Here, a technique relating to coupling of light in a grating coupler isdisclosed in “Frederik Van Laere, et. al., Journal of LightwaveTechnology, Vol. 25, No. 1, January 2007, Pages 151-156” and“Christopher R. Doerr et al., IEEE Photonics Technology Letters, Vol.22, No. 19, Oct. 1, 2010, Pages 1461-1463” (hereinafter, “Doerr”). Whena diffraction grating pitch of the grating coupler is set to d, arefractive index of the grating coupler is set to n_(eff) (=2.6), arefractive index of air is set to n_(air) (=1), an incidence angle oflight is set to Θ_(in), and a wavelength is set to λ, the relation maybe expressed as d·n_(eff)+d·n_(air)·sin Θ_(in)=λ (Equation 1). Thecoupling efficiency of the grating coupler which is expressed byEquation 1 is disclosed in (1) of Doerr, and light having the wavelengthλ satisfying Equation 1 can be coupled to the grating coupler with themaximum efficiency. Meanwhile, the reason why the sign before sin Θ_(in)in Equation 1 is “+” is because the definition of an incidence angleΘ_(in) is different from that of (1) of Doerr, but both the equationsare equivalent to each other in terms of expression.

FIG. 10 is a diagram illustrating a relationship between the incidenceangle Θ_(in) of the light incident on the grating coupler and thewavelength λ. A curve illustrated in FIG. 10 shows a relationshipbetween the incidence angle Θ_(in) and the wavelength λ based onEquation 1, and the incidence angle Θ_(in) and the wavelength λ in eachof the above-mentioned four light beams substantially satisfies(Equation 1), and thus such four light beams can be coupled to thegrating coupler 72 with low loss. The four light beams having differentwavelengths and respectively emitted from the four DFB laser units 62D,62C, 62B, and 62A, which are lined up in ascending order of wavelengthsthereof, penetrate the surface of the convex lens 64 at positionsshifted from the center of the convex lens 64 in this order, and thus itis possible to cause incidence angles Θ_(in) of the four light beamsincident on the grating coupler to be different from each other, and tocause the incidence angles to be increased in order. Furthermore, theshift from the center is determined so as to substantially satisfy(Equation 1), and thus the four light beams can be coupled to thegrating coupler with higher efficiency, thereby exhibiting a pronouncedeffect. In this embodiment, a case in which four DFB laser units areincluded in the semiconductor laser element 61 has been described as anexample. However, as long as it is a case in which a plurality (two ormore) of DFB laser units are included, the present invention is notlimited thereto.

As stated above, the optical devices according to the embodiments of thepresent invention have been described. Although a DFB laser is used as alight source of the semiconductor laser element of the optical device,it is needless to say that the present invention is not limited thereto.For example, a distributed Bragg reflector (DBR) laser may be used, andthe light source may be an element in which a laser and a modulator areintegrated. Here, the modulator may be an electro-absorption (EA)modulator or an MZ modulator. In addition, in the above embodiment,silicon (Si) used as a material of the second semiconductor substrate(second substrate) has been described as the most preferable example,but it is possible to use silicon glass (SiO₂) or a general glass inaddition to a semiconductor material such as InP or GaAs. For thisreason, the “second semiconductor substrate” in the example isillustrative of the “second substrate”, and the second substrate is notlimited to a semiconductor substrate. In addition, the present inventioncan be widely applied to an optical device including a semiconductorlaser, a lens, and a grating coupler.

While there have been described what are at present considered to becertain embodiments of the invention, it will be understood that variousmodifications may be made thereto, and it is intended that the appendedclaim cover all such modifications as fall within the true spirit andscope of the invention.

What is claimed is:
 1. An optical device comprising: a first substrate,formed of a semiconductor material and having a first surface and asecond surface which is a rear surface of the first surface, the firstsubstrate comprising: a laser unit, having an active layer laminatedbetween the first surface and the second surface and emitting light intothe first substrate from the active layer; a reflecting mirror, having aplane obliquely intersecting an optical axis of light emitted from thelaser unit, provided on the first surface reflecting the light emittedfrom the laser unit and propagating toward the second surface; and aconvex lens, provided in a region on the second surface, the regionincluding an optical axis of the light reflected by the reflectingmirror; and a second substrate, having a third surface facing the secondsurface, and being provided with a grating coupler and an opticalwaveguide on the third surface thereof, the optical waveguide havinglight incident on the grating coupler propagating therethrough, whereinthe optical axis of the light reflected by the reflecting mirrorobliquely intersects an optical axis in a center of the convex lens, andwherein an optical lens of light passing through the convex lens andbeing incident on the grating coupler obliquely intersects a normaldirection of the grating coupler.
 2. An optical device according toclaim 1, wherein an angle between the optical axis of the light and anormal line of the reflecting mirror is different from 45 degrees,wherein the light is emitted from the laser unit, propagates and reachesthe reflecting mirror.
 3. An optical device according to claim 1,wherein the optical axis of the light reflected by the reflecting mirrorobliquely intersects a normal line of second surface.
 4. An opticaldevice according to claim 1, wherein the optical axis of the lightreflected by the reflecting mirror is shifted from the optical axis inthe center of the convex lens, and the light reflected by the reflectingmirror penetrates a surface of the convex lens.
 5. An optical deviceaccording to claim 4, wherein the optical axis of the light reflected bythe reflecting mirror is offset by 18 μm or less with respect to theoptical axis in the center of the convex lens.
 6. An optical deviceaccording to claim 1, further comprising an isolator which is disposedbetween the first substrate and the second substrate and on a light pathof light which reaches the grating coupler after having passed throughthe convex lens.
 7. An optical device according to claim 1, wherein thefirst substrate comprises a plurality of integrated laser element unitseach having the laser unit, the reflecting mirror, and the convex lens,wherein a plurality of integrated waveguide circuit units each includingthe grating coupler and the optical waveguide are formed on the secondsubstrate, and wherein light having passed through the convex lens ofeach of the integrated laser element units is incident on the gratingcoupler of the corresponding integrated waveguide circuit unit.